1. Field of the Invention
This invention relates to a stamper for forming a guiding groove of an optical disk substrate (or a magneto-optical recording medium substrate) and also to a method of manufacturing the same. The present invention is particularly useful for forming a land/groove recording optical disk of which both the land section and the groove section can be used for recording.
2. Related Background Art
FIG. 11 of the accompanying drawings schematically illustrates a magneto-optical recording medium prepared by forming films on a land/groove substrate. In FIG. 11, a recording track 8 located remotely as viewed from a light beam 125 incident to the recording medium is referred to as the land section, whereas a recording track 9 located closer as viewed from the light beam 125 is referred to as the groove section. In land/groove recording, the groove section operates as the guiding groove for tracking when the land track is used for signal recording/reproduction, whereas the land section operate as the guiding groove for tracking when the groove track is used for signal recording/reproduction. Thus, this recording medium can effectively be used for improving the recording density in the track direction because both the land section and the groove section that are located side by side can be used simultaneously for recording.
Since the domain wall displacement detection (DWDD) method (U.S. Pat. No. 6,027,825) is useful for improving the recording density along the linear direction, the areal recording density of a magneto-optical recording medium can be dramatically improved by combining this method and the land/groove recording compared with a conventional recording medium.
In the land/groove recording, the use of a so-called deep groove substrate (Japanese Patent Application Laid-Open No. 9-161321) having a sharply tapered land and groove as shown in FIG. 11 is effective for facilitating the domain wall displacement. The tapered section (lateral walls between the land and the groove) are practically free from deposition of magnetic film if the magnetic films are formed by using a highly directional film forming method. Then, it is possible to produce magnetic domains whose lateral walls are practically free from magnetic domain walls for each of the land and groove sections so that the track can be magnetically segmented to facilitate the domain wall displacement. The mechanical distance between the land track 8 and the groove track 9 is preferably between about 100 nm and about 300 nm, which is at least greater than the total film thickness of the magnetic films (80 nm in the embodiment).
Additionally, the lateral walls that are free from deposition of magnetic film are effective for suppressing thermal interference of the adjacent tracks and improving the resistance against the cross erasure of the tracks. Additionally, such lateral walls may give rise to an effect of suppressing cross talks between the adjacent tracks during reproduction operation in the case of domain wall displacement detection method, because it is possible not to heat the neighboring tracks above the domain wall displacement triggering temperature Ts in the reproduction operation. Then, no domain wall displacement occurs in the magnetic domains of the neighboring tracks so that normal magneto-optical reproduction operation proceeds. Significant cross talks do not take place when the length of the recording mark is made smaller than the resolution of a light spot used for the reproduction operation.
Then, due to the synergetic effect of the magnetic segmentation of tracks and the improvement in the resistance against cross erasures and the suppression of cross talks, it is possible to dramatically improve the areal recording density by combining a deep groove substrate and the domain wall displacement detection method (see, inter alia, Shiratori: xe2x80x9cRealization of a High Density Magneto-optical Disk by Using the Domain Wall Displacement Detection Methodxe2x80x9d, Bulletin of Japan Applied Magnetism, Vol. 23, No. 2, 1999, pp. 764-769).
Meanwhile, the technique of anisotropic etching is generally used for preparing a deep groove substrate that is required to have a nearly rectangular cross section. For example, Japanese Patent Application Laid-Open No. 7-161080 describes a method of manufacturing and processing a stamper for forming a land/groove substrate by reactive ion etching (RIE).
The known method of manufacturing and processing a stamper for forming a land/groove substrate will be described by referring to FIGS. 13A through 13H and FIGS. 14A and 14B of the accompanying drawings. Firstly, a synthetic quartz master substrate 1 having an outer diameter of 350 mm, an inner diameter of 70 mm and a thickness of 6 mm that has been polished to have a surface roughness of less than 1 nm is thoroughly rinsed (FIG. 13A). (1) Then, a primer and a positive photoresist 2 are sequentially applied to the surface of the synthetic quartz master substrate 1 by spin coating. Subsequently, the master substrate is pre-baked in a clean oven. The resist has a thickness of about 200 nm (FIG. 13B). (2) Thereafter, a predetermined area of the master substrate is exposed to a laser beam emitted from a cutting machine having an Ar ion laser having a wavelength of 458 nm as a light source with a constant track pitch. In FIG. 13C, reference numeral 3 denotes the laser beam emitted from the cutting machine and reference numeral 4 denotes the exposed area, while reference number 5 denotes the unexposed area. The master substrate is continuously exposed to the laser beam typically by selecting the intensity of the laser beam so as to have a track pitch of 1.6 xcexcm and a land (or groove) width of about 0.8 xcexcm after the development process. During the exposure, the synthetic quartz master substrate is driven to rotate at a rate of 450 rpm and the laser beam spot has a diameter of 1.3 xcexcm (FIG. 13C). (3) Thereafter, the exposed areas are removed by spin development, using an inorganic alkali developing solution. Then, the master substrate is washed by means of a pure water shower, spin-dried and post-baked in a clean oven for post processing (FIG. 13D). (4) Thereafter, the master substrate is placed in a chamber of a reactive ion etching system, and after evacuating the chamber to a degree of vacuum of 1xc3x9710xe2x88x924 Pa, the master substrate is subjected to a reactive ion etching process by introducing CHF3 gas with a gas flow rate of 6 sccm, a gas pressure of 0.3 Pa, an RF power supply rate of 300 W, a self bias voltage of xe2x88x92300V and a gap of 100 mm separating the electrodes. The etching process is conducted until a predetermined groove depth (e.g., 85 nm) is obtained by regulating the etching time (FIG. 13E). (5) Then, the master substrate 7 is immersed in aremover solution prepared by mixing concentrated sulfuric acid and hydrogen peroxide to remove the remaining resist. In FIG. 13F, reference numeral 8 denotes a land section and reference numeral 9 denotes a groove section (FIG. 13F). (6) After rinsing, the surface of the master substrate 7 is turned electroconductive by forming a Ni film 10 on the surface by sputtering (FIG. 13G). (7) Then, the master substrate 7 is subjected to an electroforming process, using Ni. In FIG. 13H, reference numeral 11 denotes an electroformed Ni layer (FIG. 13H). (8) After polishing the electroformed Ni surface, the electroformed Ni layer 11 is removed from the master substrate 7 (FIG. 14A). (9) Now, a finished stamper 12 is finally produced (FIG. 14B). The same land/groove pattern can be copied to the surfaces of a number of glass substrates typically by means of a photopolymer (2P) method, using the finished stamper.
Japanese Patent Application Laid-Open No. 6-258510 discloses a mold for reproducing a diffraction grating by using a reactive ion etching technique and a method of manufacturing such a mold. FIG. 19 of the accompanying drawings is a schematic perspective view of part of the mold for preparing a diffraction grating according to the above patent document, which mold comprises a quartz-made substrate 201 having a flat surface 201a and first through third double layer films 202a through 202c formed on the surface 201a. Each of the double layer films 202a through 202c is formed by laying a pair of thin films 203, 204 of respective materials that are different from each other. For reproducing a diffraction grating a groove 205 that serves recesses having a bottom surface 205a and a pair of steps 205b, 205c is formed by removing a predetermined portion of the double layer films 202athrough 202c. The materials of the two thin films 203, 204 of each of the double layer films 202a through 202c are so selected that one of them is highly reactive to a specific etching gas (e.g., CF4) whereas the other scarcely reacts with it, while the former is scarcely reactive to another etching gas (e.g., CCl4) whereas the latter easily reacts with it. Thus, if the film thicknesses of the two thin films 203, 204 of each of the double layer films 202a through 202c are highly precisely controlled, the bottom surface 205a and the two steps 205b, 205c of the groove 205 can be made to have predetermined respective depths h1 through h3 by using the two etching gases alternately in the etching process. The etching time and other etching parameters do not need to be controlled highly accurately in this etching process and therefore the etching process can be simplified to reduce the cost of manufacturing a mold for reproducing a diffraction grating.
However, the known method of manufacturing a stamper for forming an optical disk substrate is accompanied by the following problems.
To begin with, a first problem will be discussed by referring to FIGS. 15 and 16 of the accompanying drawings. FIG. 15 is an enlarged partial view of the master substrate in the step of FIG. 13E of the reactive ion etching process. If the groove has a depth exceeding 100 nm, reactive ions are reflected by the lateral walls of the groove and condensed at the edges of the groove as shown in FIG. 15 to produce excessively etched areas as shown in FIG. 16. If a substrate is formed by using such stamper by a 2P method or an injection molding method, the groove of the substrate will have corresponding projections along the edges thereof. In the case of a deep groove substrate having a groove depth of about 150 nm, the projections may be as high as several nanometers to remarkably damage the smoothness of the groove. Then, if a domain wall displacement detection method is used, the magnetic domain walls are prevented from moving smoothly by the projections.
Now, a second problem will be discussed by referring to FIGS. 17 and 18 of the accompanying drawings. Generally, it is difficult to control the selective etching ratio of a resist and quartz in a reactive ion etching operation. The above cited Japanese Patent Application Laid-Open No. 7-161080 teaches that the controllability of the selective etching ratio of resist 2 and quartz substrate 1 is improved by appropriately selecting etching gases and using low gas pressure and low RF power. However, with such measures, the selective etching ratio is improved only to several to 1 and the applied resist can retreat as a result of the etching process. In many cases, moreover, the retreat of resist does not occur evenly Ad due to uneven exposure and the unevenness of the material of the resist so that the applied resist shows undulations 14 as shown in FIG. 17. The undulations 14 are maintained throughout the etching process to cause wrinkled surface roughness 15 on the lateral walls. The master substrate 7 also has the surface roughness 15 as shown in FIG. 18 after removing the resist, which surface roughness 15 is then transferred to a stamper to be prepared and then to a substrate prepared by using the stamper. When a substrate having such rough lateral walls is used for magneto-optical recording/reproduction, the spot of light to be used for reproduction operation is dispersed by the rough surfaces of the lateral walls to cause fluctuation of the quantity of the reflected light, which by turn increases substrate noises in the information reproduction signal and degrades the S/N ratio of the signal. Particularly, in the case of using a domain wall displacement detection method, there arises a problem that the rough surface area with wrinkles of about several tens of nanometers that are coused at each shoulder of the land section obstructs smooth movement of magnetic domain walls in addition to the problem of degraded S/N ratio.
A third problem will be discussed below. The method described in the above cited Japanese Patent Application Laid-Open No. 7-161080 comprises an etching process using CHF3 gas so that consequently fluorine resins [xe2x80x94(CF2xe2x80x94CF2)nxe2x80x94] and the like are produced abundantly during the etching process. While such resins cover the surface of the resist applied to the substrate and protects the resist against reactive ions to improve the selective etching ratio if produced at an appropriate rate, they will hardly be removed out of a deep groove to give rise to an unevenly etched surface, adversely affecting the rectangularity of the lateral walls and consequently aggravating the surface roughness of the groove sections, if they are produced excessively. These defects are transferred to the stamper and then to the substrate products reproduced by using the stamper so that, if such a substrate is used for magneto-optical recording/reproduction, the spot of light to be used for reproduction is dispersed by the rough surfaces of the lateral walls to cause fluctuation of the quantity of the reflected light, which by turn increases substrate noises in the information reproduction signal and degrades the S/N ratio of the signal.
Now, a fourth problem will be discussed below. Generally, with reactive ion etching, it is difficult to rigorously control the groove depth, that the bottom surfaces of the grooves have unevenness in terms of the groove depth due to variation in the material of the substrates and fluctuation of the atmosphere in the etching chamber. For example, the groove depth can vary by as much as 7% for the grooves having a depth of about 150 nm when using a xcfx86200 mm substrate.
Finally, a fifth problem is that the known method of manufacturing a stamper for forming an optical disk substrate requires the use of a homogeneous and costly synthetic quartz substrate.
In view of the above identified problems of the prior art, it is therefore the object of the present invention to provide a stamper that can be used for manufacturing a high precision and high performance optical disk with a simple way at low cost and a method of manufacturing such stamper.
The inventor of the present invention found that a process of forming a plurality of different thin film layers (that are different from each other in terms of etching ratio) on a master substrate as a multilayer structure or forming a thin film layer of a material different from the material of the master substrate (in terms of etching ratio) on the master substrate and then selectively etching the thin film layer(s) (anisotropic etching) is highly effective for a method of manufacturing a stamper for copying a land/groove recording optical disk. The present invention is based on this finding.
Thus, according to the present invention, there is provided a method of manufacturing a stamper for forming an optical disk substrate by applying a photoresist onto a master substrate, exposing the photoresist to light to produce a pattern, and forming a guiding groove by etching using the remaining photoresist after developement as a mask, the method comprising the steps of forming in advance a plurality of thin film layers of mutually different materials on the master substrate in a multilayer structure and sequentially etching the plurality of thin film layers selectively to produce the guiding groove.
In another aspect of the invention, there is provided a method of manufacturing a stamper for forming an optical disk substrate by applying a photoresist onto a master substrate, exposing the photoresist to light to produce a pattern, and forming a guiding groove by etching using the remaining photoresist after development as a mask, the method comprising the steps of forming in advance at least one thin film layer composed of a material different from the material of the master substrate and anisotropically etching the thin film layer selectively to produce the guiding groove.
In still another aspect of the invention, there is provided a stamper for forming an optical disk substrate manufactured by the manufacturing method according to the invention.